This invention relates to nitrogen oxide (NOx) emission controls for combustion systems such as boilers, furnaces, incinerators and other large combustion systems (collectively referred to herein as “boilers”). In particular, the invention relates to reduction of NOx emissions by selective reduction of nitrogen oxides to molecular nitrogen.
Emissions of smoke from boilers are eliminated or at least greatly reduced by the use of overfire air (OFA) technology. OFA stages the combustion air such that most of the air flows into a primary combustion chamber of the boiler and a portion of the combustion air is diverted to a burnout zone, downstream of the flame. OFA air facilitates combustion of smoke particles and smoke particle precursors.
Other types of air pollutants produced by combustion include oxides of nitrogen, mainly NO and NO2. Nitrogen oxides (NOx) are the subject of growing concern because of their toxicity and their role as precursors in acid rain and photochemical smog processes. There is a long felt need for cost-effective techniques to reduce NOx emissions generated by boilers.
A conventional NOx reduction technique is Selective Non-Catalytic Reduction (SNCR) that injects a nitrogen agent into the flue gas under conditions that cause a noncatalytic reaction to selectively reduce NOx to molecular nitrogen. The NOx reduction is selective because much of the molecular oxygen in the flue gas is not reduced. In SNCR, a nitrogen bearing reagent, e.g. HN3, urea, or an amine compound, is injected into the flue gas stream at a temperature optimal for the reaction of NH2 and NH radicals with NO reducing it to molecular nitrogen. The optimum temperature for such reactions is centered at approximately 1800° F. At substantially higher temperatures, the reagent can be oxidized to NO. At substantially lower temperatures, the reagent may pass through the flue gases unreacted, resulting in ammonia slip. An optimal range of temperatures to reduce NOx using SNCR methods is narrow and generally about 1600° F. to about 2000° F., wherein “about” refers to a temperature difference of plus or minus 25 degrees.
Flue gases reach temperatures well above 2000° F., but cool as they flow through the boiler. To allow the flue gases to cool before the nitrogen agent is released, schemes have been developed to inject relatively large droplets or particles of the agent into the flue gas, such as with the overfire air. The large droplets and particles are sized so as to release the nitrogen agent after the flue gas has cooled. See U.S. Pat. No. 6,280,695. The large droplets delay the release of the reagent in the flue gas stream until the bulk temperature of the flue gas cools to a temperature window of about 1600° F. to 2000° F.